cpufreq: intel_pstate: Adjust _PSS[0] freqeuency if needed

cpufreq: intel_pstate: Adjust _PSS[0] freqeuency if needed

The maximum turbo P-State used by the intel_pstate driver may be
limited by ACPI _PSS table entry 0.  After commit d2d22baa7a1a
(cpufreq: intel_pstate: Enforce _PPC limits), the maximum performance
on servers will be capped by the _PSS table entry 0 by default.

Even though that is formally correct, it may lead to preformance
regressions in some cases.  Namely, if the _PSS table entry 0 is
not the maximum turbo P-State, performance measured after commit
d2d22baa7a1a will not match the performance measured before that
commit on the same system.

For this reason, modify the code to always use the maximum turbo
frequency as the one that corresponds to _PSS table entry 0 if turbo
is enabled in the BIOS.  This way, the performance levels from
before commit d2d22baa7a1a will be restored on the affected systems.

Fixes: d2d22baa7a1a (cpufreq: intel_pstate: Enforce _PPC limits)
Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
[ rjw : Changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>